1. Field of the Invention
The present invention relates to a distance measuring apparatus for measuring a distance to a target by using pulse light.
2. Related Background Art
It has been known that optical systems for distance measuring apparatus using light are roughly classified into two kinds, i.e., biaxial and coaxial types, depending on the arrangement of their light-transmitting optical path through which measurement pulse light is propagated and light-receiving optical path through which a reflected beam of the measurement pulse light is propagated. The light-transmitting optical path and the light-receiving optical path constitute a measurement optical path. Of these two types of optical systems, the coaxial optical system is advantageous from the viewpoints of reducing the size of apparatus, eliminating parallax, and making the light-transmitting optical path coincide with a collimation optical system for focusing onto a target. It is due to the fact that the coaxial optical system has a configuration in which the light-transmitting axis for transmitting light to the target and the light-receiving axis for receiving the reflected beam from the target coincide with each other.
Also, this kind of distance measuring apparatus includes those employing a continuous modulation system in which a light source such as LED, semiconductor laser, or the like is continuously modulated, thereby the distance to the target is determined from the phase difference between the transmitted light and the received light; and those employing a pulse system in which a semiconductor laser is used as a light source, thereby the distance to the target is determined from the period of time between the time at which pulse light is transmitted and the time at which part of thus transmitted light is returned after being reflected by the target.
In order to respond to the demand for enhancing the range capable of distance measurement and the demand for the ability of non-prism measurement in which reflectors such as corner cube reflector are not used at the measuring point for saving labor and attaining efficient operation, the pulse system adapted to employ a large peak power is quite advantageous.
Meanwhile, in the distance measuring apparatus using a coaxial optical system, when the output of the light source is intensified in view of the demand for enhancing the range capable of distance measurement and the demand for the ability of non-prism measurement, reflections within the apparatus such as those at the inner side face of an objective lens and the inner face of a lens barrel may be received by a light-receiving device as flare, thereby yielding a large error in measured values. Though the pulse system is quite advantageous for the demand for enhancing the range capable of distance measurement and the like, the pulse light has a greater peak, thus increasing the magnitude of flare, thereby yielding a larger error in measured values in the pulse system.
Methods for eliminating the error in measured values caused by flare are disclosed, for example, in U.S. Pat. No. 4,113,381 and Japanese Utility Model Publication No. 3-21502. These methods employ a configuration in which the emission optical system and the light-receiving optical system are optically separated from each other, so as to eliminate flare.